This invention relates to a method for purifying a crude diphenylsulfone tetracarboxylic acid.
Certain processes have been proposed for preparing diphenylsulfone tetracarboxylic acids (hereinafter referred to as "DSTA"). For example, USSR Patent No.422,730 discloses a process for producing DSTA by oxidizing 3,3',4,4'-tetramethyldiphenylsulfone with molecular oxygen in the presence of a heavy metal catalyst such as a cobalt salt. In practicing such process for an industrial purpose, it becomes necessary to separate the heavy metal catalyst used and undesired byproducts from the resultant crude DSTA in order to recover the expensive heavy metal and to obtain purified DSTA free from the contamination by the heavy metal and byproducts. While these processes give DSTA unavoidably contaminated with impurities such as catalytic components, byproducts and the like, no method has been proposed, nevertheless, for effectively purifying crude DSTA to obtain highly purified DSTA.